Geothermal heating/cooling systems using heat ground source energy receive the worldwide application.
The problems of enlargement of Earth source exchanger productivity by the way of increasing of heat transfer between ground and heat exchanger liquid are known. From the point of view of maximum heat transfer, vertical exchange geothermal systems are the most studied and the most proposed structures relate to them. U.S. Pat. No. 7,370,488 proposes effective vertical geo-thermal heat exchanging system providing “the transfer of heat energy using coaxial-flow heat exchanging structures installed in the earth for introducing turbulence into the flow of the aqueous-based heat transfer fluid flowing along the outer flow channel”
Patent Application US20110308268 A1 describes a vertical underground exchanger, which comprises an internal cylinder with low heat conductivity (HDPE polymer) and external thin wall stainless steel cylinder with high heat conductivity coaxially installed in a bore. The lower ends of the cylinders have bottoms, the bottom of the internal cylinder has holes for water circulation. The gap between the cylinders is filled by sand. During operation, circulating water from the heat pump flows from the top of the cylinder column through the saturated sand medium in a U-configuration at the desired pump rate. The slow passage of water, coaxial with the thin stainless steel cylinder wall together with the special conductivity characteristics of water saturated sand assures efficient ground to water heat transfer. The improved heat transfer results in greater operating efficiencies for geothermal heating and cooling. However, firstly, the described structure comprising two concentric cylinders with sand filling of gap between them and water entrance and flow through sand in outer cylinder and exit in the inner cylinder really is acceptable for vertical (not horizontal) exchangers only, secondly, water flow through sand due to significant hydraulic resistance requires spending considerable capacity that decreases the system efficiency.
U.S. Pat. No. 6,251,179 proposes for vertical geothermal heat pump systems with high density polyethylene (HDPE) piping with circulating water or water/antifreeze liquid to use thermally conductive cementitious grout for boreholes filling Grout 111. Grout 111 or analogous grouts were proposed for DX (direct exchanger) geothermal systems (U.S. Pat. No. 7,856,839, U.S. Pat. No. 7,938,904) with copper pipes. According to U.S. Pat. No. 6,251,179 heat conductivity of wet Grout 111 is 2.42 W/m*K and dry Grout 111 is 2.16 W/m*K.
The above patents relate to vertical geothermal systems providing good heat transfer between the ground and heat medium. However, vertical systems are considerably more expensive than horizontal geothermal heat exchangers. Besides, vertical boreholes have depth 15-120 m with corresponding disadvantage of deep drillings, considerably complicated installation and maintenance. Vertical drilling can provoke mixing between aquifers of different qualities and to be potential source of contamination. Before application of vertical geothermal system geological survey is required. For drilling of boreholes and mounting of exchanging system special equipment is required. Vertical collectors are used where land area is limited and for larger power installations.
U.S. Pat. No. 7,942,015 describes horizontal underground exchanger based on application of rainfall or other precipitation collecting sump in trench with impermeable member on bottom of the trench for trapping water. The trench is filled with different dimensions particulate material through which heat exchange pipes pass. Unidirectional water permeable layer is located on top of sump, directly under road covering.
The system described in U.S. Pat. No. 7,942,015 has the following shortcomings:                This structure is applicable as geothermal system in regions with significant rainfall and other precipitation.        If impermeable member does not reach the top of sump with the unidirectional water permeable layer, in arid and semiarid regions surrounding dry soil will absorb the water, filling sump will be dried and will have low thermo-conductivity. Using of this system artificially filled with water will require large amount of water consumption.        If impermeable member reaches the top of sump with the unidirectional water permeable layer and “heat exchange pipe is buried approximately 1.5 meters below the surface”, sump is located from said depth up to ground surface. Due to increased thermo-conductivity of the sump filling, the temperature inside the sump will not correspond to the temperature at depth of Earth 1.5 meters, but will rise in the summer and fall in winter, i.e. will vary with varying temperature of air and ground surface. However, the main geothermal principle is: ground temperature near the heat exchange pipes must be close to stable in all seasons, time of day, weather, etc. It means that in this system there is no ability for effective using of geothermal energy.        When impermeable member reaches the top of sump without the unidirectional water permeable layer, in addition to temperature instability there is necessity in too much quantity of water that can't be provided in arid and semiarid regions.        
Patent EP2418439 describes horizontal geothermal system, in which underground exchanger consists of metal or plastic box in a form of parallelepiped with internal inserts, which form internal cavity inside the box, and the boxes are connected hydraulically in series. All connected inserted boxes are installed in trench. Such construction appears expensive and complicated.
Other systems, which contain underground heat exchangers (but not Geothermal ground-coupled heat exchanger), are known as Pit Thermal Energy Storages (See, for example, Michael Harris “Thermal Energy Storage in Sweden and Denmark”, 3.4.4 Pit Thermal Energy Storage (PTES), available on the Internet at lup.lub.lu.se/luur/download?func=downloadFile&recordOId=2174449&fileOId=2174452
PTES is used for storing maximal solar thermal production (in summer). PTES are storages of thermal energy seasonally in large pits, which are usually dug into the ground, lined with an impermeable plastic barrier and filled with water or water with gravel. PTES act like a large accumulator tank and, as a rule, are insulated from ground and air. In summer water in the pit is warmed by heat source. In the winter the thermal energy is extracted by the heat pump and used in the district-heating network. “Properly insulated, the elevated temperature is capable of being maintained from season to season”.
Another article (see F. Ochs, W. Heidemann, H. Müller-Steinhagen, H. Koch, “Soil-water pit heat store with direct charging system”
available on the Internet at intraweb.stockton.edu/eyos/energy_studies/content/docs/FINAL_PAPERS/13A-2.pdf considers structures of seasonal thermal storages including pits with water, gravel and saturated soil layers. The pit is insulated from Earth on all sides: from the bottom, walls, and lid. Solar collectors are used as heat source during the day.
Thus, the water in the pit is heated by external heat source (solar water heating panels, power plants), but not by energy of the Earth. Heat exchange with the Earth in this case means the loss of energy, which shortens the storage time of the previously stored heat energy.
U.S. Pat. No. 4,466,256 describes ground-installed coldness storage in a form of a pit with water-soaked soil separated from Earth by impermeable plastic material, wherein a given volume of the water-soaked soil is frozen in winter by passing an air-chilled heat-exchange liquid through tubes buried in this water-soaked soil. Frozen medium is used in summer for cooling. As it is written in the patent, the pit with frozen medium is insulated from the Earth due to “self-insulating effect of melting water which inevitably occurs after initial freezing” (U.S. Pat. No. 4,466,256, column 5, lines 40-42). This self-insulating effect considerably decreases heat exchange process with Earth. Thus, frozen matrix due to “effect to insulate the frozen mass” principally cannot be used in geothermal systems.
Besides, in U.S. Pat. No. 4,466,256 frozen matrix top is located at depth 0.3-0.4 meters, where stable Earth temperature required for geothermal heat exchanger is not provided.
There is one more property of all Pit Thermal Energy Storages, which prevents using of them in geothermal systems. This is a fundamentally for energy storage that decreasing of ratio surface area/volume of pit reduces heat losses. In geothermal systems ratio surface area/volume must be maximal as possible for effective geothermal heat exchange process. Therefore, requirements to the form of pits and geothermal systems are opposite.
Thus, requirement for PTES to minimize energy loss through heat exchange with Earth leads to the fundamental structure differences between two systems—Pit Thermal Energy Storages and Geothermal ground-coupled heat exchanger,
Horizontal heat exchangers are the simplest and cheapest for geothermal application. Trenching costs for horizontal loops usually are much lower than well-drilling costs for vertical closed-loops, and there are more contractors with the appropriate equipment and experience.
But the main disadvantages of horizontal systems are larger land area required, dependence of performance on season, rainfall, drought potential (low groundwater levels), especially in sandy soils and elevated areas.
The proposed geothermal horizontal exchanger structures allow to decrease at 2-4 times the required land area and to make possible effective their application in arid and semi-arid climate zones.